lxc-alpine: use dl-cdn.a.o as default mirror instead of random one

Some mirrors from the mirrors list are not very reliable and it seems
that no one really wants to use some random mirror as the default
option.

Signed-off-by: Jakub Jirutka <jakub@jirutka.cz>

templates/lxc-alpine.in

@@ -46,7 +46,7 @@ ebf31683b56410ecc4c00acd9f6e2839e237a3b62b5ae7ef686705c7ba0396a9  alpine-devel@l
 12f899e55a7691225603d6fb3324940fc51cd7f133e7ead788663c2b7eecb00c  alpine-devel@lists.alpinelinux.org-5261cecb.rsa.pub"
 
 readonly APK_KEYS_URI='http://alpinelinux.org/keys'
-readonly MIRRORS_LIST_URL='http://rsync.alpinelinux.org/alpine/MIRRORS.txt'
+readonly DEFAULT_MIRROR_URL='http://dl-cdn.alpinelinux.org/alpine'
 
 : ${APK_KEYS_DIR:=/etc/apk/keys}
 if ! ls "$APK_KEYS_DIR"/alpine* >/dev/null 2>&1; then
@@ -76,7 +76,7 @@ usage() {
 		                          to the host arch.
 		   -d, --debug            Run this script in a debug mode (set -x and wget w/o -q).
 		   -F, --flush-cache      Remove cached files before build.
-		   -m URL --mirror=URL    The Alpine mirror to use; defaults to random mirror.
+		   -m URL --mirror=URL    The Alpine mirror to use; defaults to $DEFAULT_MIRROR_URL.
 		   -r VER, --release=VER  The Alpine release branch to install; default is the
 		                          latest stable.
 
@@ -130,11 +130,6 @@ parse_arch() {
 	esac
 }
 
-random_mirror_url() {
-	local url=$(fetch "$MIRRORS_LIST_URL" | shuf -n 1)
-	[ -n "$url" ] && echo "$url"
-}
-
 run_exclusively() {
 	local lock_name="$1"
 	local timeout=$2
@@ -477,7 +472,7 @@ extra_packages="$@"
 # Set global variables.
 readonly DEBUG="$debug"
 readonly FLUSH_CACHE="$flush_cache"
-readonly MIRROR_URL="${mirror_url:-$(random_mirror_url)}"
+readonly MIRROR_URL="${mirror_url:-$DEFAULT_MIRROR_URL}"
 
 # Validate options.
 [ -n "$name" ] || die 1 'Missing required option --name'